Vertices deforming although ungrouped (weight painting)
I checked the file: nothing is wrong in the hand bones, but remember that if mesh deforms something else must have weights for it.

In this case is (at least) another bone - maybe you selected it inadvertently while assigning weights to the hand mesh.

See how: you start with this



but moving "UpperLeg.R" bone, you see how hand fingers deform



so while you move the "intended bone" (Hand.R), the distance form "UpperLeg.R" changes and thus the deform influcence, so you get those strange deforms...

hth